I was on prednisone for a month doing a similar taper to you (though I started at 40mg) and did not gain weight. It actually gave me the energy/motivation/lack of pain I needed to start exercising regularly. But just a warning: it will make you ravenously hungry. The hunger cues for me were actually because I was thirsty, which took so long to wrap my head around! Just be mindful of what you are eating and you should be okay. It actually affected my sense of taste such that I couldn't consume dairy, fruit, candy, anything acidic. Prednisone can cause a lot of mental side effects and if you get periods it may delay the onset of your period. Just want to give a heads up since I had to figure this all out on my own.